Carbon assimilation variation and control in Picea rubens, Picea mariana, and their hybrids under ambient and elevated CO2

After 3 years of CO (2) treatments, A stimulation from ambient to elevated CO (2) was strongly related to the total dry mass change (%), supporting the sink demand A hypothesis. Adaptations related to gas exchange are important fitness traits in plants and have significant growth and ecological implications. Assimilation (A) and assimilation to internal CO2 (AC (i) ) response curve parameters were quantified from a red spruce (RS) (Picea rubens Sarg.)-black spruce (BS) [P. mariana (Mill.) B.S.P.] controlled-cross hybrid complex grown under ambient and elevated CO2 conditions. Under ambient conditions, maximum A (A (max)), maximum rate of carboxylation by rubisco (V (cmax)), maximum rate of electron transport (J (max)), and carboxylation efficiency (CE) generally increased with increasing BS content; however, under elevated CO2 conditions, hybrid index 50 (hybrid index number is the percentage of RS, balance BS) often had greater values than the other indices. There were significant hybrid index, CO2, and hybrid index x CO2 effects for A (growth) at 360 ppm (A (360)) and 720 ppm (A (720)). The net A stimulation (A (stim)), from ambient to elevated CO2 treatment after 3 years was 10.8, 57.8, 74.1, 69.8, and 58.7 %, for hybrid indices 0 (BS), 25, 50, 75, and 100 (RS), respectively. Why does BS have the least A (stim), hybrid index 50 the most, and RS a moderate level? There was a significant relationship between A (360) and ambient total biomass among indices (P = 0.096), but none was found between A (720) and elevated total biomass. However, A (stim) (%) was strongly related to the change in total dry mass (%) in response to elevated CO2 (R (2) = 0.931, P = 0.008), supporting the hypothesis that sink demand drives A. Traits A (max), V (cmax) and J (max) were correlated to total chlorophyll concentration. Moreover, A (max) V (cmax) and J (max) also showed a significant underlying male effect, particularly under ambient conditions consistent with the paternal inheritance of the chloroplast genome.